The present invention relates to devices for sensing vibrations in earth formations. More specifically, the present disclosure is directed to electrodynamic sensing devices, such as geophones and seismometers, that have a moving coil placed in a magnetic field in a centered position. The present disclosure may be applicable to other types of vibration transducers, either in sensing or transmitting operation.
In the oil and gas industry seismic sensors are deployed at various locations, such as on the earth surface, in the sea, at the seabed, or in a borehole, to provide operationally significant subsurface structural and material information by measuring seismic signals reflected from changes in the subsurface structures. In this, seismic sensors are commonly used for purposes of obtaining useful data relating to acoustic impedance contrasts in subsurface structures.
Seismic sensors are also prevalent in earthquake monitoring, long term monitoring for water and CO2 reservoirs, nuclear test monitoring, and such like activity that require the accurate and efficient acquisition of seismic data.
In seismic signal detection, the vibrations in the earth resulting from a source of seismic energy are sensed at discrete locations by sensors, and the output of the sensors used to determine the structure of the underground formations. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic signals from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. For example, the sensed seismic signals may be direct signals that are derived from micro-seismicity induced by fracturing or reservoir collapse or alteration, or reflected signals that are derived from an artificial source of energy.
Sensors fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense particle motion arising from a seismic source.
As depicted in FIG. 1A, a typical geophone 10 has one or more cylindrical moving coil 12 that is suspended by springs 20 so as to be disposed around a magnet 15 having pole pieces 16. The geophone 10 has a housing 14 and end caps 18. Each moving coil 12 is maintained at a neutral, rest position by the springs 20, and is free to oscillate in a magnetic field of the magnet 15 from a centered position thereof. Springs 20 are usually made with a sheet metal designed to maintain the coil 12 at a centered, equilibrium position relative to the magnetic field of the magnet 15. In a geophone that is designed for vertical operation, the springs 20 are pre-stressed to centralize the moving coil 12 in the magnetic field against gravitational acceleration.
When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth's surface, in the sea or at the seabed, or on the wall of a borehole which penetrates the earth, moves with the particle motion caused by acoustic wave propagation.
If the axis of the geophone is aligned with the direction of motion, however, the moving coil mounted on the spring inside the geophone stays in the same position causing relative motion of the coil with respect to the housing. When the coil moves in the magnetic field, a voltage is induced in the coil which can be output as a signal.
FIG. 1B is a schematic depiction of a geophone in which x0 is the neutral position of the moving coil, x is the position of the coil in motion and ξ is the relative displacement of the coil against the center of the magnetic field. The spring and mass system creates a natural frequency, ω0=√{square root over (k/m)}, where k is the spring constant and m is the moving mass of the coil assembly. The movement of the moving coil relative to the magnetic field generates an electric output
      e    g    =            S      0        ⁢                  ⅆ        ξ                    ⅆ        t            where S0 is the sensitivity and
      ⅆ    ξ        ⅆ    t  is the velocity of the coil above the natural frequency of the geophone. The generated electric signal flows through the shunt resistor Rs and coil. The current i in the coil damps the movement of the coil.
In seismicity monitoring, it is desirable to measure the position or displacement of the seismic sensor moving coil relative to the magnetic field in the seismic sensor housing. Co-pending, commonly owned, U.S. patent application Ser. No. 12/471,467, titled “Methods and Systems for Seismic Signal Detection”, describes in detail the importance and use of displacement data in seismicity monitoring.
As described in the aforementioned patent application, it is possible to lower the natural frequency of a geophone by using positive displacement feedback. Additionally, displacement and velocity signals may be combined to obtain a wide frequency response. Furthermore, it is possible to use calibration to determine feedback parameters and to equalize the geophone response by adding the integral of displacement, i.e., an open loop control.
As further described in the aforementioned patent application, borehole geophones are expected to work under tilt since a borehole can be deviated. However, if a geophone is tilted, i.e., is moved away from the orientation that it is designed for, the pre-stressed springs cause the moving coil to move in the upward direction. Therefore, the moving coil is displaced from its neutral position relative to the vertical position of the geophone.
Furthermore, after installation in a deep hole a geophone may be required to continuously monitor seismicity for many years. The geophone is expected to function reliably for a long time at high temperatures. However, over time there is creep in the springs due to the affect of high temperatures. Since spring creep causes the coil to be displaced from the center over time the geophone response also changes.
When the moving coil is not centered in the magnetic flux field, the open circuit sensitivity, S0 and open circuit damping, D0 are reduced and total harmonic distortion becomes large. In this, if a vertical geophone is tilted from its vertical position the geophone response parameters So, Do, and fo change based on the amount of tilt. Changes in geophone response parameters change the waveform of recorded seismic signals, which is not desirable for the analysis of the recorded data.
The aforementioned patent application provides solutions to the problems discussed above relating to tilted geophones and spring creep. As also noted therein, it is desirable to determine the amount of displacement of the moving coil of a geophone in order to compensate for tilt and spring creep using electrical levitation.
Applicant further recognized that it is often desirable to measure low frequency signals, for example, to study the source mechanism of an earthquake or the seismicity as a result of fault movement. In this, the scale of a fault slip may be misread because of the insensitivity of a seismic sensor at low frequencies. A seismometer having a simple structure is similar to a geophone with a large moving coil and a large spring to reduce the natural frequency. However, the displacement response of the moving mass of such a geophone is large, and the velocity is small at low frequencies. Therefore, for low frequencies it is desirable to measure the displacement of the moving coil because the electrical signal is large.
In the past, moving coil displacement data has been acquired by, for example, providing an additional position sensor in the seismic sensor. However, extraction of displacement signals from a moving mass of the seismic sensor poses problems with respect to the wiring since there is one pair of springs that typically is used for the output of velocity signals from the moving coil of the seismic sensor. In addition, as discussed in further detail below, conventional mechanisms such as pigtail connectors are not suitable for the purposes described herein.
The limitations of conventional seismic sensor designs noted in the preceding are not intended to be exhaustive but rather are among many which may reduce the effectiveness of previously known sensor mechanisms. The above should be sufficient, however, to demonstrate that sensor structures existing in the past will admit to worthwhile improvement.